1. Field of the Invention
The present invention relates to semiconductor devices and more particularly to a semiconductor device having a surge protection circuit.
2. Description of the Background Art
There have been proposed various surge protection circuits for protecting, a motor vehicle, a motor, a fluorescent display and an audio device, for example, as well as an IC (Integrated Circuit) constituted of transistor devices for example, from an instantaneously increased current or voltage (surge). One of the proposed circuits, a surge protection circuit constituted of one diode and one npn transistor is known as a device which can be implemented with a simple configuration. A conventional surge protection circuit constituted of one diode and one npn transistor is configured as detailed below.
In the conventional surge protection circuit constituted of one diode and one npn transistor, the cathode of the diode has a first n+ diffusion layer of high concentration formed at a main surface of a semiconductor substrate that is electrically isolated by a field oxide film. The first n+ diffusion layer contacts a conductive layer formed on the semiconductor substrate and is thus connected electrically to a signal input terminal. The anode of the diode has a p-type diffusion layer and a p+ diffusion layer formed in the p-type diffusion layer. The p+ diffusion layer directly contacts the n+ diffusion layer serving as the cathode.
Moreover, the collector of the npn transistor has the first n+ diffusion layer, a buried n+ diffusion layer and an n− epitaxial layer formed in the semiconductor substrate. The base of the npn transistor has the p-type diffusion layer formed in the n− epitaxial layer. The emitter of the npn transistor has a second n+ diffusion layer formed in the p-type diffusion layer.
The first n+ diffusion layer is included in the cathode region of the diode and included in the collector region of the npn transistor. Further, the p-type diffusion layer is included in the anode region of the diode and in the base region of the npn transistor.
An operation of the above conventional surge protection circuit is now described. When a surge voltage is applied to the signal input terminal, the surge voltage is then applied to the first n+ diffusion layer to increase a reverse voltage of the diode. This reverse voltage exceeding a certain level causes Zener breakdown of the diode and accordingly a current flows from the cathode to the anode of the diode. The p-type diffusion layer included in the anode region is also included in the base region of the npn transistor. Then, the current flows as a base current of the npn transistor. Accordingly, the npn transistor is turned on to discharge the charge of the surge applied to the signal input terminal from the emitter of the npn transistor.
Any surge protection circuits except for the above one are disclosed, for example, in Japanese Patent Laying-Open Nos. 5-206385 and 56-19657.
In the conventional surge protection circuit having the above-described configuration, the first n+ diffusion layer is formed to have a high concentration in order to reduce contact resistance between the first n+ diffusion layer serving as the cathode of the diode and the conductive layer mentioned above. Further, if a part of the anode region that contacts the first n+ diffusion layer has a low concentration, electrons that are present in a depletion layer of the pn junction between the first n+ diffusion layer and the anode (p-type diffusion layer) are trapped in the field oxide film adjacent to the first n+ diffusion layer when breakdown of the diode occurs. Then, a problem arises that the depletion layer of the pn junction expands to increase the breakdown voltage of the diode. Therefore, it is required to form the p+ diffusion layer of high concentration in a part, which contacts the first n+ diffusion layer, of the p-type diffusion layer serving as the anode, in order to flow electrons from the first n+ diffusion layer smoothly to the anode (p-type diffusion layer). In other words, both of the anode region and the cathode region constituting the pn junction where the Zener breakdown occurs should be formed with high concentration.
If, however, the anode region and the cathode region forming the pn junction where Zener breakdown occurs are both formed with high concentration, the width of the depletion layer of the pn junction between the anode region and the cathode region is extremely small. A phenomenon (leakage of current) then occurs that is current flow in the surge protection circuit with any voltage lower than the breakdown voltage, resulting in a problem that the surge protection circuit does not operate normally.